12 THE VOYAGE OF H.M.S. CHALLENGER. 



phenomena are shown in their simplest form, whilst at Batavia the more striking effects 

 of the influence of land are apparent. 



In order to show the constancy, or otherwise, of the times of occurrence of the 

 maxima and minima, a series of twelve maps of the globe were prepared for the month 

 of June, showing at all stations from which the required data have been obtained, the 

 deviations at noon, 2 p.m., 4 p.m., etc., G.M.T., from the daily mean pressure; and 

 thence four lines were drawn showing the places where, at that hour, the maxima and 

 minima occurred. For fully 30° north and south of the equator the lines of maxima 

 and minima ran north and south, but in higher latitudes these lines are changed, 

 particularly as regards the forenoon maximum and the afternoon minimum. For 

 example, at 6 p.m. the line indicating the afternoon minimum is for the latitude of 

 London in long. 16° W. ; in lat. 30° N., it is in long. 35° W., in which meridian it holds 

 its course southwards as far as lat. 30° S. ; its course thence turns south-westwards to 

 near the Falkland Islands, long. 60° W. It follows that in June the afternoon minimum 

 occurs about three hours earlier in the Falkland Islands than in the south-west of Ireland, 

 thus showing in a striking manner the influence of season on the diurnal phenomena of 

 pressure. In cases where the lines of maxima and minima cross such regions as southern 

 and western Europe, whose surface is diversified by large tracts of land and sheets of 

 water, the deflexions are peculiarly striking and instructive.. 



In middle and higher latitudes in summer, proximity to the sea, conspicuously so 

 when the station is situated on the west coasts of continents and islands, delays the 

 time of occurrence of the morning maximum and the afternoon minimum ; whilst in 

 continental situations the morning maximum occurs much earlier than in lower latitudes, 

 and the evening minimum nearly as late as at places near the sea. It appears from 

 the Challenger observations that these peculiarities of the curves do not occur over the 

 open sea in the higher latitudes. 



The retardation of the time of occurrence of the morning maximum is greatest in 

 situations which, while strongly insular in character, are at the same time on, or not 

 far from, an extensive tract of land to eastward or south-eastward. This is well 

 illustrated by the hourly oscillations of the barometer for the year at Helder, in the 

 north-west of Holland, and Sitka, in the south-east of Alaska (Table IV., App. pp. 28 

 and 36). The deviations from the means are given in thousandths of an inch. 



It is seen that at Helder, the morning maximum occurs at times varying from nine 

 to ten o'clock in the beginning of the year, and successively later as the year advances, 

 till in June it is delayed to 2 p.m., and thereafter it occurs earlier and earlier month by 

 month till January, when it is at the earliest. The following selected cases of the 

 hourly deviations of pressure in June illustrate the gradual occurrence earlier of this 

 phase according as the place becomes less insular as described above, the series closing 

 with Kew and Culloden, to which are added Katherinenburg and Fort Piae. A selection 

 of these is plotted on Plate I., figs. 8-15. 



